At 1:40 on August 16, 2016, the Long March II carrier rocket was launched from the Jiuquan Satellite Launch Center for the 29th time and carried the world's first quantum science experimental satellite "Mozi", which successfully sent the artificial satellite it carried. Entered the scheduled track. So what is quantum communication? What is the significance of the quantum satellite launching?
The development of quantum communication
The advent of quantum communication systems has ignited hopes of building "absolutely secure" communication systems. At present, the practical and industrialization of quantum communication has become the goal of major powers to pursue.
In the international race of quantum communication, China is a latecomer. After years of hard work, China has become one of the world's leading experts in quantum information research, and is also in the forefront of the world's quantum communication technology. It has completed the construction of large-scale quantum communication metropolitan area networks such as Hefei and Jinan, and the “Beijing-Shanghai trunk lineâ€. The large-scale fiber-optic quantum communication backbone network is also about to be completed.
However, this is only the beginning. "In the city, building a metronium quantum communication network through fiber optics is the best solution. But to achieve long-distance and even global quantum communication, relying solely on fiber-optic quantum communication technology is not enough." Academician Pan Jianwei, chief scientist of quantum satellites, said.
He explained that because the quantum information carrier photons can only reach the final receiving station after a hundred kilometers of propagation in the optical fiber, it is difficult to break through the quantum communication of the optical fiber to reach the order of 100 kilometers. However, after the photon penetrates the entire atmosphere, it can retain about 80%. Then, using the satellite's transit, it can realize wide-area quantum secure communication on the ground thousands of kilometers away or even covering the whole world.
In addition, the wonders of the centuries-old quantum theory have been repeatedly tested in the laboratory, but have never been verified on the space scale. Are the various singular phenomena of quantum theory still present in space? Can quantum entanglement and teleportation extend between the stars and the earth? These all require satellites to verify.
In 2011, the Chinese Academy of Sciences officially launched the development of the world's first "quantum science experimental satellite", which means that Chinese scientists are the first to challenge the satellite communication of Xingdi, which means that China will lead the European and American countries to obtain the ability of quantum communication to cover the whole world.
About the quantum satellite "Mozi"
"Mozi No." is one of the first experimental satellites of the space science pilot project of the Chinese Academy of Sciences. The main scientific goal is the Xingdi high-speed quantum key distribution experiment. Based on this, the wide-area quantum key network is experimentally developed in order to practicalize space quantum communication. It will distribute entangled photons in space, experiment with quantum teleportation, and test the completeness of quantum mechanics at spatial scales.
The original concept of quantum satellite began more than a decade ago. In 2001, 31-year-old Pan Jianwei returned from Europe and established a quantum information laboratory at the Chinese University of Science and Technology. In 2003, when most people were still working on the principle demonstration in the laboratory, Pan Jianwei and his colleagues had already developed the initial concept of the “world-integration†quantum communication network. The “quantum science experimental satellite†is exactly this concept. The key node.
Around this vision, the Pan Jianwei team began a technical accumulation of more than ten years. He led the team to achieve international leading breakthroughs in free space quantum key distribution, quantum entanglement distribution and quantum teleportation experiments.
In 2005, the team of Pan Jianwei realized the 13 km free space quantum communication experiment in the world for the first time. It was confirmed that the photon can be effectively maintained after the photon penetrates the atmosphere, thus verifying the feasibility of the quantum communication.
Subsequently, they continued to create "legends": 16-kilometer free space quantum teleportation, 100-kilometer free-space quantum communication, and terrestrial quantum communication's all-round ground-validation experiments... laid a solid foundation for Xingdi Quantum Communication.
Pan Jianwei said that after the launch of the "Mozi No.", if the effect is expected, the next step is to launch "Mozi No. 2" and "Mozi No. 3". “A single low-orbit satellite cannot cover the whole world, and due to the strong solar background, the current satellite-to-ground quantum communication can only be carried out at night. To achieve efficient global quantum communication, a satellite network needs to be formed.â€
In the future, a "comet group" consisting of dozens of quantum satellites will "join hand" with the ground quantum communication trunk to support the quantum communication network of "integration of heaven and earth."
By 2030, China will strive to build a global wide-area quantum secure communication network. On this basis, the "quantum Internet" with sufficient information security is constructed to form a complete quantum communication industry chain and the next generation national sovereign information security ecosystem.
Mozi's "four weapons" challenge four major experimental tasks
At present, no country in the world has sent quantum science experiments into space. The development of quantum satellites has no experience to follow, and the process is full of difficulties and challenges. Scientists carried the "four weapons" independently developed on the Mozi quantum satellite: quantum key communication machine, quantum entanglement transmitter, quantum entanglement source and quantum test control and processor.
At the same time, the scientific application system was built on the ground, including one center--Hefei Quantum Science Experiment Center; four stations--Nanshan, Delingha, Xinglong, Lijiang Quantum Communication Ground Station; one platform-Ali Quantum Teleportation experiment platform.
Together with the ground station, the satellite and the ground station form a quantum science experimental system. During the two-year design life, four experimental tasks will be carried out—the Xingdi high-speed quantum key distribution experiment, the wide-area quantum communication network experiment, and the star-ground quantum entanglement distribution. Experiment, geostationary quantum teleportation experiment.
Pan Jianwei introduced that the experiment is roughly divided into three categories: the first is to carry out quantum key distribution between satellite and ground to realize secure communication between heaven and earth, if any of the four ground stations can achieve secure communication between two. The network can be realized; the second type is equivalent to moving the quantum laboratory into space, and testing quantum theory at the spatial scale; the third is the realization of quantum-state stealth transmission of the scale of the satellite and the ground.
The Tiandi Quantum Science experiment is very complicated, and the requirements for the world's experimental equipment are also unusually high. Pan Jianwei admits that the most difficult part of the satellite development process is the payload, "to overcome many technical problems before winning." For example, the quantum entanglement source, which only has the size of the set-top box, is very important, and it can generate entangled light, which is the source of various experiments of quantum satellites in the air. In the usual laboratory, the volume of the entangled source is very large. The researchers not only made it small, but also realized it to meet the requirements of the space environment through a series of innovations, which was realized for the first time in the world.
Zhu Zhencai, chief of the Quantum Satellite System, introduced that in the quantum satellite flight, the two lasers carried must be aimed at two ground stations that are thousands of kilometers apart, and transmit the quantum key to the left and right simultaneously, and the optical axis and ground telescope on the satellite. The optical axis should always be precisely aligned, just like the "needle tip" on the satellite to the "male" on the ground.
The research team carried out various experiments to test the ultra-long-distance "moving aiming target" capability, and finally broke through the key technologies such as the alignment of the satellite light path. Through the two-stage control of the platform and the load, the alignment precision can reach 10 of the ordinary satellite. Times.
"The laser has been used by one station for one station, but one satellite has been aligned with two ground stations. It has never been international. If it is successful, it is the first time in the world to achieve such high-precision tracking and ground station cooperation." Wu Ji, director of the National Space Science Center, said.
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